Apparatus for reducing polymerization inhibitor in recycle streams

ABSTRACT

Methods and apparatus are disclosed for removing undesirable volatile components having high boiling points from a product stream during polymerization of monovinyl aromatic compounds such as styrene, which methods include the use of heat exchangers, devolatilizers, and filter beds.

FIELD OF THE INVENTION

This invention relates to apparatus for polymerizing monomers such asmonovinyl aromatic compounds, including styrenes, alpha-methyl styrenes,and ring-substituted styrenes; and more particularly involves methodsand apparatus for removing acid species and other undesirable compoundsfrom the recycle subsystems in polymerization systems.

BACKGROUND OF THE INVENTION

In the field of polymers, one of the most widely manufactured classes ofpolymers is that encompassing polymerized monovinyl aromatic compoundssuch as styrenes, alpha-methyl styrenes, and ring-substituted styrenes.Of this class, the most common members are normal polystyrene, andrubber-modified polystyrene--often called high impact polystyrene orHIPS.

In the continuous process of polymerizing styrene monomer intopolystyrene and HIPS, a common process of carrying out thepolymerization is in reactor vessels, using polymerization initiatorssuch as t-butyl peroxybenzoate and dibenzyl peroxide. One disadvantageof using such initiators is that a by-product of the polymerizationprocess using these initiators is acid decomposition by-products, suchas benzoic acid, which acidic species then react with the initiator and,as a consequence, inhibit further polymerization, when recycled withunreacted monomer back into the polymerization reactors.

One method of handling this problem is that disclosed in U.S. Pat. No.4,857,587 to Sosa, et al, the entire disclosure of which is herebyincorporated by reference into this application. In the Sosa patent,recycled unreacted monomer is passed through a recycle treatment vesselto remove acidic species. The vessel would normally contain an adsorbentmaterial such as alumina or clay to remove a major portion of the acidcomponent from the recycle stream.

Other methods and apparatus for reducing the effect of aciddecomposition by-products are disclosed in the two Sosa et al patents,U.S. Pat. Nos. 4,777,210 and 4,861,827, the disclosures of which arehereby incorporated by reference herein. The '210 patent discloses apreinversion reactor to control particle size and the '827 patentteaches the use of initiators which do not decompose into acidby-products during the polymerization process.

The present invention provides an advancement over the known methods andapparatus for minimizing acid decomposition by-products, whichadvancement can also be used in conjunction with the above-describedpatented apparatus to even further reduce unwanted by-products.

SUMMARY OF THE INVENTION

The present invention comprises apparatus for reducing acidicdecomposition by-products in the recycle stream of polystyrene andmodified polystyrene polymerization systems which apparatus utilizes asecond devolatilizer in the process stream to concentrate the removal ofunwanted acidic decomposition by-products.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic flow diagram illustrating the presentinvention in place in a typical high-impact polystyrene polymerizationplant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the FIGURE, there is illustrated therein a schematicrepresentation of a series of reaction vessels and associated apparatususeful in the practice of styrene polymerization and more particularlyin the production of rubber-modified styrene using the presentinvention.

In the manufacture of HIPS material using this system, styrene,polybutadiene, a free-radical initiator, and additional components suchas solvents, anti-oxidants, and other additives are usually fed to apolymerization reactor indicated at CSTR through a feed line or multiplefeed lines generally indicated at F1. The polymerization reactor CSTR isof the type commonly referred to as a continuous stirred tank reactor.As used herein, the term "styrene" includes a variety of substitutedstyrenes, e.g. alpha-methylstyrene, ring-substituted styrenes, such asp-methylstyrene and p-chlorostyrene, as well as unsubstituted styrene.Typically, the mixture in polymerization reactor CSTR will compriseabout 75 to 99% by weight styrene, about 1 to 15% by weightpolybutadiene, about 0.001 to 0.2% by weight free-radical initiator, andabout 0.1 to about 6% by weight additional components.

The reactor CSTR, as previously mentioned, is a continuously stirredtank reactor which operates at a percent solids level above theinversion of the polymer system. That is, the polymerization reactoroperates at a percent solids level such that the system has a continuousphase of polystyrene and a discontinuous phase of dispersed droplets ofrubber; or preferably, the droplets are a mixture of polystyrene andrubber.

The CSTR reactor is sometimes also referred to as a "boiling reactor"which means that the styrene/polystyrene/rubber mixture therein isallowed to "boil" by vaporization of the lighter components (styrenemonomer, ethylbenzene, for example). This vaporizing of lightercompounds serves to remove a large amount of the heat of polymerization,and allows the operator to better control the rate of polymerization inthe CSTR vessel.

The vapor components from the CSTR reactor are drawn off of the vesseland piped via line F13 to the main condenser as described hereinbelow,there to be condensed for return to the reactor via the recycle stream.

Preferably, the apparatus used in practicing the present invention mayadditionally comprise a second polymerization reactor of the continuousstirred tank reactor type which is operated at a preinversion condition,i.e., the continuous phase therein is a styrene-rubber solution and thediscontinuous phase is polystyrene. The preinversion reactor (not shown)is usually located directly ahead of, or upstream of, the polymerizationreactor CSTR such that the styrene, polybutadiene, free-radicalinitiator, and other components are fed to the preinversion reactorfirst and the mixture exiting the preinversion reactor is then fed tothe CSTR reactor. The preinversion reactor is also preferably acontinuous stirred tank reactor.

The liquids/solids output from the polymerization reactor CSTR is fed toanother polymerization reactor through flow line F2 where post-inversionstage polymerization occurs. Preferably this next polymerization reactoris a linear-flow reactor, such as a plug flow reactor, but may also be atower-type reactor or other known reactor configuration. The figureindicates a single linear-flow reactor PFR, whereas more than onelinear-flow reactor may be utilized, placed in serial connection withthe PFR reactor. This achieves increased polymerization in eachsubsequent horizontal reactor. The output from the plug flow horizontalpolymerization reactors is at a temperature of around 340° F. and isdirected to a preheater PH and from there to a conventionaldevolatilizer DV1 through flow lines F3 and F4, respectively. Thetemperature of the material leaving the preheater is preferably in therange of about 480° F. Unreacted styrene monomer and other volatileresidual components leave devolatilizer DV1 through overhead line F5 asa recycle stream component. This recycle stream is preferably returnedto the present system after condensation in condenser COND and furthertreatment in clay bed treater CBT via flow line F6. The treated recycledstream then flows through flow line F7 back to the polymerizationreactor CSTR. Periodically the absorbent material, whether it be clay oralumina, or other suitable material, in the treater CBT is backflushedand regenerated, with the waste material being transported via flow lineF8 to waste storage tank WST.

Referring back to the output of devolitilizer DV1, in addition to theoverhead recycle stream containing the aforementioned volatilecomponents flowing through F5, the polymerization reactants, at about99.5% solids and comprising polymerized high impact polystyrene andnonvolatile components at a temperature of about 440° F., flow out thelower end of DV1 through flow line F9 to a second devolatilizer DV2. Indevolatilizer DV2 additional volatile components are separated andflowed out through line F10, and finished polystyrene or high impactpolystyrene moves through product line F11 at a temperature of about440° F. to a pelletizer or other type of finished product formulationunit.

The devolatilizers DV1 and DV2 preferably have conventional heatingelements, such as external heater coils, arranged to maintain thereactant stream therein at a relatively constant temperature in therange of about 440° F.

The devolatilizers use a combination of the heat already in thematerial, heat added by the heater coils, and a vacuum in the range ofabout 1 to 10 mm Hg to vaporize or "boil-off" the volatiles in thestream. Preferably the first devol unit DV1 maintains a low pressure(vacuum) of around 5-10 mm Hg and the second devol a lower pressure(higher vacuum), in the range of only about 1 mm Hg.

This higher pressure (lower vacuum) in DV1 then preferentially vaporizesthe desirable lower boiling point volatiles such as styrene monomer,ethylbenzene, and other aromatics and vinyl structures. On the otherhand, the greater vacuum in DV2 allows the undesirable, higher boilingpoint volatiles to be boiled off therein, such as the oxygenatedspecies, acids, quinones, phenols, etc.

The volatile components from DV2 flowing through line F10 are thenpassed through a total condenser TC which totally condenses thesevolatile components by means of a cooling medium CM flowing thereacross,and these condensed components are then flowed through waste line F14 towaste sump W. A pump P then pumps the waste components through line F12to the waste storage tank WST.

The recycle streams coming off of the devolatilizers DV1 and DV2 containa variety of impurities. The major impurities in these streams can betraced to products of reactions between species necessarily present inthe recycle stream, such as styrene monomer and anti-oxidant, impuritiesfrom the rubber, and unwanted species in the systems, such as oxygen.Although some of the recycle stream impurities are innocuous, it hasbeen unexpectedly discovered that certain impurities in the recyclestream adversely affect the polymerization process and/or the resultantHIPS product when the recycle stream is introduced back into the system.

In the continuous process performed by the apparatus of the presentinvention, polymerization of the styrene monomer is initiated by thedecomposition of a free-radical initiator. Initiating radicals for thepolymerization reaction are generated by the decomposition of thefree-radical initiator into one or more primary radicals. The primaryradical then reacts with styrene monomer to initiate polymerization.

Typically, the free-radical initiator is fed to the first polymerizationreactor CSTR which is maintained at conditions under which thefree-radical initiator decomposes, although it may also be provided tothe aforementioned preinversion reactor ahead of the CSTR reactor or itmay be introduced at the linear flow reactor PFR. The free-radicalinitiator may alternatively be selected such that it will not decomposein the first polymerization reactor and rather will decompose under theconditions maintained in some subsequent polymerization reactor such asthe PFR or subsequent linear flow reactor. In this case, polymerizationof styrene monomer in polymerization reactors can be thermallyinitiated. Alternatively, a combination or two or more free-radicalinitiators can be used, such that one free-radical initiator decomposesin the polymerization reactor CSTR and another free-radical initiatordecomposes in the linear flow reactor PFR.

Decomposition of the free-radical initiator, which initiatespolymerization of the styrene monomer, also produces decompositionby-products which do not participate in the polymerization reaction. Inthe present continuous flow process, these decomposition by-products ofthe free-radical initiator are removed from the HIPS polymer in thedevolatilizers DV1 and DV2 and, unless removed from the recycle stream,would be reintroduced into the polymerization reactors.

Investigation of the effects of various recycle stream components uponstyrene polymerization has shown that acid decomposition by-products offree-radical initiators react with such initiators, thereby inhibitingstyrene polymerization. It is believed that these acidic decompositionby-products adversely affect free-radical initiator efficiency byinducing decomposition of the free-radical initiator and/or trappingfree-radicals produced by spontaneous (as opposed to induced)decomposition of the free-radical initiator. Thus, the acidicdecomposition by-products decrease the number of free-radicals availableto initiate polymerization of the styrene monomer, which in turndecreases the efficiency of the free-radical initiator.

Benzoic acid is one example of an acid decomposition by-product havingsuch an adverse effect. The realization that benzoic acid in the recyclestream inhibits styrene polymerization in the presence of free-radicalinitiators is particularly significant since benzoic acid is adecomposition by-product of t-butyl peroxybenzoate and dibenzylperoxide, two of the most commonly used free-radical initiators in thecontinuous process production of HIPS. Benzoic acid is also producedfrom the air oxidation of benzylaldehyde, which in turn is produced fromthe oxidation of styrene. Other acidic species may be present in thepolybutadiene rubber. It is well known that phenolic anti-oxidants,sulphur components, and substituted phosphites are added to protect therubber from oxidation. The aforementioned incorporated patent U.S. Pat.No. 4,857,587 lists several acid-producing free-radical initiators andtheir corresponding acid decomposition by-products. Also shown thereinis a graphical illustration of the detrimental effects of benzoic acidon styrene polymerization when using the free-radical initiator t-butylperoxybenzoate.

According to the process of the aforementioned incorporated '587 patent,the adverse effects of acid decomposition by-products of free-radicalinitiators are avoided by directing the recycle stream through a recycletreatment vessel CBT interposed between the devolatilizer DV1 and therecycle stream return line F7. The recycle treatment vessel CBT,preferably comprises at least one adsorbent material, such as alumina orclay, which is capable of removing acid components from the recyclestream. Other examples of specific adsorbent materials include alumina,attapulgus clay, carbon, silica gels, and Porocel (trademark for analumina). The size and shape of the recycle treatment vessel isdetermined according to standard engineering practices. Preferably theCBT is a clay-filled tower, maintained at approximately 80° to 120° F.and about 20 to 25 psi pressure.

While the recycle treatment vessel CBT must be capable of removingsubstantially all of the acid components from the recycle stream, it ishighly desirable that the adsorbent used also be capable of removingother impurities, both identified and unknown, from the recycle stream.The combined effects of all impurities, including acid decompositionby-products, upon styrene polymerization reaction rate and the averagerubber particle size in the resultant HIPS polymer are significantlydetrimental, and increase with an increase of the amount of impurities.

In addition to the removal of some of the acid by-products species fromthe volatile line off of devolatilizer DV1, it has further been foundthat by proper selection of vacuum levels in the devolatilizers, alarger percentage of the undesirable products can be made to pass outthrough product line F9 from DV1 and into the second devolatilizer DV2where those undesirable volatile components can be removed from thepolymerized material thereby preventing overburdening of the clay bedtreater while obtaining a further protection of the initial free-radicalinitiators.

In the present invention, the major portion (approximately 80%) of thevolatile components leading from the preheater PH are removed indevolatilizer DV1 and passed through flow line F5. Approximately 20% ofthe removable volatiles are then removed from the polymer stream indevolatilizer DV2. Originally the volatile components from DV2 wereadded back into the recycle stream F5 through flow line F10. It has beenfound however that a major portion of the undesirable acidic by-productsare removed in the volatiles of DV2 rather than DV1. More than 80% ofthe total volatiles are removed in DV1 and added to the recycle streamand less than 20% of the total volatiles are removed from DV2, yet thelow volatile output of DV2 comprises a major portion of the acidicby-products which destroy the free-radical initiators. Therefore, it wasfound that rather than adding the output of devolatilizer DV2 back intothe recycle stream, the clay bed treatment vessel life can be extendedsignificantly by routing the volatiles from DV2 into a total condenserTC through flow line F10, totally condensing all of the volatiles, andremoving them to a waste sump W whereupon they are pumped by pump P tothe waste storage tank WST.

Thus, the loss of less than 20% of the volatiles from the recycle streamresults in removal of more than half of the undesirable aciddecomposition by-products from the recycle stream, which greatly extendsthe life of the adsorbent material in the treatment vessel CBT.

The present invention comprises apparatus for manufacturing polymerizedstyrenics, which utilizes a combination of devolatilizers to removevolatile components from the polymerized styrene material and filter thevolatile components to remove destructive acid by-products. The systemencompasses a second devolatilizer downstream of a first devolatilizerfor selectively removing undesirable volatile components from thefinished polystyrene and, instead of recycling these undesirablevolatile components, condensing them in a total condenser and pumpingthem to a waste storage tank. The removal of all of the volatilecomponents from the second devolatilizer results in a loss of less than20% of the total volatiles but also results in removal of more than halfof the destructive acid by-product components and a resulting decreasein the burden on the filter treatment vessel CBT.

Although the invention has been described with reference to particularembodiments thereof, it will be apparent to those skilled in the artthat various changes and modification can be made without departing fromthe spirit of the invention or from the scope of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Apparatus for purifyingmonovinyl aromatic streams in a polymerization process, said apparatuscomprising:a monovinyl aromatic polymerization reactor system adapted topolymerize monomeric monovinyl aromatic feedstock; a vapor line comingoff of said reactor system for bleeding off vaporized volatiles in saidreactor system and for removing heat of polymerization from said reactorsystem; a condenser arranged to receive and condense said vaporizedvolatiles from said vapor line; a filter system arranged to receive saidcondensed volatiles and further arranged to filter undesirable compoundsfrom said condensed volatiles; a recycle line leading from said filter,communicating filtered condensate back to said reactor system; a firstdevolatilizer arranged to receive the monovinyl aromatic stream fromsaid reactor system and adapted to vaporize lower boiling pointvolatiles from said stream and communicate said vaporized volatiles tosaid condenser; a final devolatilizer arranged to receive said monovinylaromatic stream and adapted to vaporize higher boiling point volatilestherefrom; and, a vapor line leading from said final devolatilizer andarranged to conduct said higher boiling point volatiles away from saidreactor system.
 2. The apparatus of claim 1 wherein said firstdevolatilizer comprises a heated vacuum vessel arranged to maintain thematerial therein at around 440° F. and at a pressure of about 5-50 mmHg; and said second devolatilzer comprises a heated vacuum vesselarranged to maintain the material therein at around 440° F. and at apressure of about 0.5-5 mm Hg.
 3. A monovinyl aromatic polymerizationsystem comprising:a reactor assembly arranged to polymerize monovinylaromatic monomer, a condenser assembly arranged to receive and condensevolatiles from said reactor assembly; a filter arranged to filterundesirable volatiles from said condensed volatiles and return saidcondensed volatiles to said reactor assembly; a first devolatilizercommunicating with said reactor assembly, arranged to receive theproduct stream therefrom, and adapted to further vaporize lower boilingpoint volatiles from said product stream; a vapor line leading from saidfirst devolatilizer to said condenser assembly for transferringvaporized volatiles therebetween; a second devolatilzer communicatingwith said first devolatilizer, arranged to receive said product streamtherefrom, and adapted to vaporize higher boiling point volatiles fromsaid product stream; and, a vapor line on said second devolatilizerarranged to transfer said higher boiling point volatiles away from saidpolymerization system.